The present invention relates to liquid crystal visual display units making it possible to display visual information represented by an electrical signal applied to said units. It also relates to telephone terminals incorporating such a unit and which make it possible, for example, to remotely consult a telephone directory.
It is known to construct liquid crystal visual display units using different electro-optical effects.
U.S. Pat. No. 4,202,010 corresponding to French Pat. No. 2389955 entitled "Picture display unit and television system using such a unit" describes a visual display unit in which inscription takes place by means of a thermoelectric effect.
For this purpose the unit comprises a system of heating lines which are successively excited by a heating voltage. The liquid crystal passes into a liquid phase along the line which is heated and then becomes diffusive on cooling. The unit also comprises a system of columns which intersect with the system of lines. A group of voltages representing the line to be inscribed is applied to these columns during the cooling of the line. As a function of the value of the electrical field resulting from the voltages applied the liquid crystal becomes more or less diffusive at each intersection between the columns and the cooling line. It is thus possible to inscribe or enter in the unit a picture or image analysed in accordance, for example, with a television field. This inscription takes place on a line by line basis. The thus inscribed picture remains stored until an erasure takes place. The latter occurs, for example, on inscribing the following picture where each line is erased during heating and then inscribed with the new signal during cooling.
The dimensions of this unit are essentially limited for thermal dissipation reasons. Thus, if it was desired to obtain a unit which could be observed with the naked eye and at a reasonable distance a very high power would have to be dissipated in each heating line. It would not be possible to control this power and it could lead to the destruction of the system. In order to observe the thus inscribed picture it is possible in certain cases to use a magnifying glass, but most often it is projected on to a screen. For the use of such a unit in a television receiver the overall dimensions, cost and maintenance requirements of the ancilary devices necessary for carrying out such a projection are of little importance.
However, when using such a unit in a telephone terminal, particularly for consulting a directory, it is absolutely necessary to be able to see it directly without effecting a projection. The dimensions which are then necessary, approximately 10 centimeters, do not make it possible to construct the unit without adaptation.